Many methods for manufacturing ceramic electronic components have been known and a description is made here of a typical method of manufacturing multilayer ceramic capacitors.
First, a powder of such dielectric materials as barium titanate and the like is added with an organic binder, plasticizer, solvent and the like and the resulting mixture is kneaded and made into a slurry. Then, the slurry is coated using a doctor blade method or the like and dried to produce a ceramic green sheet. Next, a conductive paste mainly composed of a metal is printed on the ceramic green sheet by a screen printing method or the like and dried to form a conductive layer, thereby allowing an active layer sheet to be prepared. Aside from above, a cover layer sheet composed only of a ceramic green sheet, which has no conductive layer formed thereon, is prepared.
As FIG. 12 shows, according to a prior art process of stacking multilayer ceramic capacitors, adhesive layer 102 is disposed on supporting plate 101 and a plurality of cover layer sheets are stacked thereon. Further, on top of that, an active layer sheet is superimposed, thus putting together a ceramic green sheet and a conductive layer to form a stack structure. The step of superimposing an active layer sheet is repeated a predetermined number of times and further a plurality of cover layer sheets are again stacked on top of the plurality of active layer sheets to realize stack body 103. When the active layer sheets are stacked on top of each other, the stacking is performed in such a manner that a plurality of rectangular patterns of respective conductive layers, each acting as an internal electrode, are staggered alternately from layer to layer by a predetermined distance in the length direction of the rectangular pattern.
In the step of forming a high-density structure by pressing, a pressing force is applied to stack body 103 to have respective ceramic green sheets and conductive layers pressed against one another to form a one-piece structure. In the prior art step of forming a high-density structure by pressing, a uniaxial press with flat lower die 111 and flat upper die 112 arranged in parallel with each other is used. Stack body 103 as composed on supporting plate 101 via adhesive layer 102 is disposed on lower die 111 and a pressing force is applied via upper die 112 to form a high-density structure of stack body 103.
Next, the high-density structure of stack body 103 is cut into pieces, each having a desired configuration, and separated from adhesive layer 102 on supporting plate 101, to produce green chips. The green chips are sintered and external electrodes are provided on each respective chip to complete a multilayer ceramic capacitor.
The aforementioned step of forming a high-density structure by pressing is a very important step to prevent a failure due to structural defects such as delamination and the like from occurring. When the extent of adhesive joining between respective ceramic green sheets and conductive layers is insufficient, it is likely to cause a failure due to structural defects. Therefore, in order to establish the densifying condition to a sufficient extent, it is necessary for a pressing force to be uniformly and sufficiently applied to stack body 103, thereby allowing respective ceramic green sheets to be deformed in the thickness direction thereof and to be pressed against each other to realize an excellent density condition.
However, when a ceramic green sheet has a pressing force applied thereto or is exposed to a temperature and a pressure needed for densifying, the ceramic green sheet is deformed not only in the thickness direction thereof but also in the direction parallel to the surface thereof. This means that the periphery of stack body 103 tends to expand outwards, thereby causing a distortion in the shape of the conductive layer. As a result, when stack body 103 is cut into pieces, a failure due to disconnection and a failure due to poor characteristics are caused. These problems are likely to be multiplied as the step-wise difference in level due to missing of a conductive layer is increased because increasing numbers of the conductive layers are involved and/or the thickness of a ceramic green sheet is small, thereby making the ratio of the thickness of conductive layers occupying in the thickness of stack body 103 more significant.
Therefore, various proposals have been made with respect to a method for preventing the deformation of a stack body from occurring when a pressing force is applied thereto. For example, in the Japanese Patent Application Unexamined Publication Nos. H5-175072 and 2001-23844, disclosed methods include:
A) a method for forming a high-density structure by first applying a pressing force to the periphery on the surface of a stack body by means of a peripheral section die and then applying a pressing force to the inner part below the surface of the stack body by means of a central section die;
B) a method for applying a higher pressing force to a stack body by means of a peripheral section die than the pressing force applied to the stack body by means of a central section die; and
C) a method for applying a pressing force to a stack body placed in an elastic framework by means of a uniaxial rubber press.
Even according to the methods A and B, however, when the step-wise difference in level due to missing of a conductive layer is large, a plastic deformation of a ceramic green sheet takes place not only in the thickness direction but also in the direction parallel to the surface of the ceramic green sheet. As a result, the stack body expands at the outer periphery thereof to cause a distortion in the shape of the conductive layer.
Even according to the method C, it is necessary for the dimensions of the stack body to match the inner dimensions of the elastic framework with a high degree of precision and even a little difference in the dimensions allows the deformation of the stack body to occur.